There is an increasing demand for laser sources that can be tuned over a wide range of wavelengths. One application of growing importance is in multiwavelength optical networks, especially DWDM networks. Such tunable lasers can maximize the transmission capacity of multiwavelength optical networks by providing different wavelengths on demand and can improve network reliability by serving as standbys for sets of single-frequency lasers whose wavelengths fall within the tunable laser bandwidths.
Various methods of delimiting the laser optical resonance cavity use reflective optics: narrow band reflectors such as prisms, gratings and other dichroic filters or broadband reflectors such as reflective coatings and dielectric mirrors. For tuning, the characteristic reflective wavelength at one end of the laser cavity can be altered to select the lasing wavelength. A tuning range comparable to the laser gain bandwidth would permit the whole bandwidth to be utilized.
The laser cavity can be implemented in a waveguide such as an optical fiber or a planar waveguide doped with a laser amplifying element (e.g. a rare-earth element such as erbium, neodymium, ytterbium). The cavity can also be implemented in a semiconductor. Changing the length of the laser optical cavity provides wavelength-tuning of the laser.
In some applications discontinuous jumps between adjacent longitudinal modes (mode hopping) impedes smooth tuning. Smooth continuous tuning without mode hopping has been achieved by concurrently altering a wavelength dependent feedback of the resonator (refraction grating or Fabry-Perot etalon) and the length of the optical cavity. But this approach is complex and expensive to implement, involving sophisticated controls and optic mounts with close tolerances. Another attempt used piezoelectric transducers (PZTs) to strain an optical fiber comprising of two fiber Bragg gratings and an intervening rare-earth-doped region. See U.S. Pat. No. 5,317,576, issued to Frederick Leonberger et al. on May 31, 1994 and entitled "Continuouly Tunable Single-Mode Rare-Earth Doped Pumped Laser Arrangement." While this method eliminates mode hopping during tuning, the strain produced by the piezoelectric effect is relatively small, limiting the tunable range of the device (a 0.8 nm maximum shift in wavelength for a 15 cm long piezoelectric transducer). Laser gain media with bandwidths of 40-60 nm are now available, and so it is desirable to have a much broader tuning range. In addition to having a limited tuning range, a PZT also requires a sustained application of electric power with relatively high voltage, e.g. typically .about.100 volts. Magnetostrictive tuning suffers from similar drawbacks.
An alternative way to tune a laser having fiber Bragg grating reflectors is through thermal variation. The thermally-induced refractive index change in a fiber Bragg grating produce a wavelength shift of roughly 0.01 nm/.degree. C. Since the reflective properties of the grating start to degrade above 100-200.degree. C., the practical thermal tuning range is about 1 nm. In addition, thermal tuning is slow (of the order of several seconds), and also requires sustained power.
Accordingly there is a need for a laser that is quickly and latchably tunable over a large range of the gain bandwidth.